1. Field of the Invention
The present invention relates to the use of coherent energy pulses, as from high power pulsed lasers, in the shock processing of solid materials, and, more particularly, to methods and apparatus for pressure pulse confinement within overlay material during operation. The invention is especially useful for enhancing or creating desired physical properties such as hardness, strength, and fatigue strength.
2. Description of the Related Art
Known methods for the shock processing of solid materials typically involve the use of high explosive materials in contact with the workpiece. High explosive materials or high pressure gases are used to accelerate a plate that strikes the solid to produce shock waves therein. Shot peening is another widely known and accepted process for improving the fatigue, hardness, and corrosion resistance properties of materials by impact treatment of their surfaces.
Shock processing with coherent radiation has several advantages over what has been done before. For example, the source of the radiation is highly controllable and reproducible. The radiation is easily focused on preselected surface areas and the operating mode is easily changed. This allows flexibility in the desired shocking pressure and careful control over the workpiece area to be shocked. Workpieces immersed in hostile environments, such as high temperature and high vacuum can be shock processed. Additionally, it is easy to shock the workpiece repetitively.
Laser peening (hereinafter referred to as laser shock processing) utilizes two overlays: a transparent overlay (usually water) and an opaque overlay (usually an oil-based or acrylic-based black paint). During processing, a laser beam is directed to pass through the water overlay and is absorbed by the black paint, causing a rapid vaporization of the paint surface and the generation of a high-amplitude shock wave. The shock wave cold works the surface of the part and creates compressive residual stresses, which provide an increase in fatigue properties of the part. A workpiece is typically processed by processing a matrix of overlapping spots that cover the fatigue critical zone of the part.
A problem in the art of laser shock processing is the production of irregularities on the surface of a workpiece during processing. The irregularities are due to non-uniform or uneven containment of a pressure pulse applied to the surface of a workpiece. These irregularities appear as visible hemispherical bosses of unprocessed areas on the surface of workpiece.
The problem of pressure pulse containment within the transparent overlay material utilized in laser shock processing can be traced back to at least 1976. An example of a reference documenting the problem of pressure pulse containment is a French reference by P. Forget et. al., "Laser Shock Surface Treatment of Ni-Based Superalloys," Materials and Manufacturing Processes, Vol.5., No. 4, pages 501-528 (1990).
What is needed in the art is a laser shock process that is highly repeatable without irregularities in the applied pressure.